This project has as its goal the elucidation of ultraviolet mutagenesis in Escherichia coli. In its current phase, the project assumes that an inducible type of error-prone DNA repair is responsible for UV mutagenesis, and that the signal for its induction is unscheduled interruption of DNA replication. The same signal activates other functions (collectively known as "SOS" functions), including prophage induction, filamentous growth, inhibition of Exonuclease V activity and reinitiation of DNA replication at the chromosomal origin. A model has been developed to account for the coordinated regulation of SOS functions, and for their common requirement for the active products of the recA and lexA genes. According to this model, all SOS repressors are protected against inactivation by cellular proteases by the joint action of the recA and lexA gene products. We propose to test many of the predictions generated by this model. The major emphasis during the coming year will be on the isolation of mutants deficient in protease activity, or mutants in which the activity or specificity of proteases is altered. The expression of SOS functions will be carefully examined in these mutants. The map locations of these mutations will be determined, and their effects on protease activity will be characterized by biochemical methods. BIBLIOGRAPHIC REFERENCES: Witkin, E. M., Relationships among repair, mutagenesis and survival: overview. In: Molecular Mechanisms for Repair of DNA, Part A, eds. P. Hanawalt and R. Setlow, Plenum Publishing Co., New York, 1975. pp 347-353. Volkert, M. R., D. L. George and E. M. Witkin. Partial suppression of the LexA phenotype by mutations (rnm) which restore ultraviolet resistance but not ultraviolet mutability to Escherichia coli B/r uvrA lexA. Mutation Res. 36: 17-28. 1976.